Method and system for wavelength-dependent imaging and detection using a hybrid filter

ABSTRACT

Apparatus and methods for wavelength-dependent detection are provided. A detector includes a hybrid filter having unpatterned and patterned filter layers and at least one light-detecting sensor that detects light in first and second wavelength bands from the patterned filter layer of the hybrid filter. The unpatterned filter layer passes light in two nonoverlapping wavelength bands relative to light in wavelength bands between or among the nonoverlapping wavelength bands. The patterned filter layer includes first and second regions configured respectively to pass light in the first and second wavelength bands of the nonoverlapping wavelength bands passed by the unpatterned filter layer.

TECHNICAL FIELD

Embodiments in accordance with the invention relate generally to thefield of imaging. More particularly, embodiments in accordance with theinvention relate to methods and systems for wavelength-dependent imagingand detection using a hybrid filter.

BACKGROUND

There are a number of applications in which it is of interest to detector image an object. Detecting an object determines the absence orpresence of the object, while imaging results in a representation of theobject. The object may be imaged or detected in daylight and/or indarkness, depending on the application.

Wavelength-dependent imaging is one technique for imaging or detectingan object, and typically involves detecting one or more particularwavelengths that reflect off, or transmit through, the object. In someapplications, only solar or ambient illumination is required, while inother applications additional illumination is needed. Light istransmitted through the atmosphere at many different wavelengths,including visible and non-visible wavelengths. Thus, the wavelengths ofinterest may not be visible.

FIG. 1 is a diagram of the spectra of solar emission, a light-emittingdiode, and a laser. As can be seen, the spectrum 100 of a laser is verynarrow, while the spectrum 102 of a light-emitting diode (LED) isbroader in comparison to the spectrum of the laser. And solar emissionhas a very broad spectrum 104 in comparison to both the LED and laser.The simultaneous presence of broad-spectrum solar radiation can makedetecting light emitted from an eyesafe LED or laser and reflected offan object quite challenging during the day. Solar radiation can dominatethe detection system and render the relatively weak scatter from theeyesafe light source small by comparison.

Additionally, the object being detected may not remain stationary duringsuccessive measurements. For example, if a human being is the object,the person may shift position or move during the time the measurementsare taken. If measurements made at different wavelengths are made atdifferent times, movement of the object during successive measurementscan distort the measurements and render them useless.

SUMMARY

In accordance with the invention, a method and system forwavelength-dependent imaging and detection using a hybrid filter isprovided. An object to be imaged or detected is illuminated by a singlebroadband light source or multiple light sources emitting light atdifferent wavelengths. The light is detected by a detector, whichincludes a light-detecting sensor covered by a hybrid filter. The hybridfilter includes a multi-band narrowband filter mounted over a patternedfilter layer. The light strikes the narrowband filter, which passeslight at or near the multiple wavelengths of interest while blockinglight at all other wavelengths. The patterned filter layer alternatelypasses the light at one particular wavelength while blocking light atthe other wavelengths of interest. This allows the sensor to determineeither simultaneously or alternately the intensity of the light at thewavelengths of interest. Filters may also be mounted over the lightsources to narrow the spectra of the light sources.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will best be understood by reference to the followingdetailed description of embodiments in accordance with the inventionwhen read in conjunction with the accompanying drawings, wherein:

FIG. 1 is a diagram of the spectra for solar emission, a light-emittingdiode, and a laser;

FIG. 2 is a diagram of a system for pupil detection that utilizes ahybrid filter in an embodiment in accordance with the invention;

FIG. 3 a illustrates an image generated with an on-axis light source inaccordance with the system shown in FIG. 2;

FIG. 3 b depicts an image generated with an off-axis light source inaccordance with the system shown in FIG. 2;

FIG. 3 c illustrates an image resulting from the difference between theFIG. 3 a image and the FIG. 3 b image;

FIG. 4 depicts a sensor in one embodiment in accordance with theinvention;

FIG. 5 is a cross-sectional diagram of a detector in accordance with oneembodiment in accordance with the invention;

FIG. 6 depicts spectra for the polymer filters and the narrowband filtershown in FIG. 5;

FIG. 7 a illustrates a first method for fabricating a dual spike filterin an embodiment in accordance with the invention;

FIG. 7 b depicts the spectrum for the dual spike filter shown in FIG. 7a;

FIG. 8 a illustrates a second method for fabricating a dual spike filterin an embodiment in accordance with the invention;

FIG. 8 b depicts the spectrum for the dual spike filter shown in FIG. 8a;

FIG. 9 is a diagram of a light source and a narrowband filter in anembodiment in accordance with the invention;

FIG. 10 illustrates the spectra of the light source and the combinationof the light source and the narrowband filter, shown in FIG. 9;

FIG. 11 is a diagram of a second system for pupil detection thatutilizes a narrowband filter with each light source and a hybrid filterwith a sensor in an embodiment in accordance with the invention;

FIG. 12 is a diagram of a device that can be used for pupil detection inone embodiment in accordance with the invention;

FIG. 13 is a diagram of a system for detecting transmission through anobject that utilizes a hybrid filter in another embodiment in accordancewith the invention;

FIG. 14 depicts spectra for polymer filters and a tri-band narrowbandfilter in an embodiment in accordance with the invention;

FIG. 15 depicts a sensor in accordance with the embodiment shown in FIG.14;

FIG. 16 depicts spectra for polymer filters and a quad-band narrowbandfilter in an embodiment in accordance with the invention; and

FIG. 17 depicts a sensor in accordance with the embodiment shown in FIG.16.

DETAILED DESCRIPTION

The following description is presented to enable one skilled in the artto make and use the invention, and is provided in the context of apatent application and its requirements. Various modifications to thedisclosed embodiments will be readily apparent to those skilled in theart, and the generic principles herein may be applied to otherembodiments. Thus, the invention is not intended to be limited to theembodiments shown, but is to be accorded the widest scope consistentwith the appended claims and with the principles and features describedherein. It should be understood that the drawings referred to in thisdescription are not drawn to scale.

Embodiments in accordance with the invention relate to methods andsystems for wavelength-dependent imaging and detection using a hybridfilter. A technique for pupil detection is included in the detaileddescription as one such system that utilizes a hybrid filter inaccordance with the invention. Hybrid filters in accordance with theinvention, however, can be used in a variety of applications wherewavelength-dependent detection and/or imaging of an object or scene isdesired. For example, a hybrid filter in accordance with the inventionmay be used to detect movement along an earthquake fault, or to detectthe presence, attentiveness, or location of a person or subject.Additionally, a hybrid filter in accordance with the invention may beused in biometric applications, such as, for example, systems thatidentify individuals using their eyes or facial features.

With reference now to the figures and in particular with reference toFIG. 2, there is shown a diagram of a system for pupil detection thatutilizes a hybrid filter in an embodiment in accordance with theinvention. The system includes a detector 200 and two light sources 202,204. The system may also optionally incorporate a controller orprocessor (not shown) in other embodiments in accordance with theinvention.

Light sources 202, 204 are shown on opposite sides of detector 200 inthe FIG. 2 embodiment. In other embodiments in accordance with theinvention, light sources 202, 204, may be located on the same side ofdetector 200. Light sources 202, 204 may also be replaced by a singlebroadband light source emitting light at two or more differentwavelengths in other embodiments in accordance with the invention. Oneexample of such a broadband light source is the sun.

In this embodiment for pupil detection, two images are taken of asubject's 206 face and/or eyes using the detector 200. One of the imagesis taken using light source 202, which is close to or on the axis 208 ofthe detector 200 (“on-axis”). The second image is taken using lightsource 204 that is located at a larger angle away from the axis 208 ofthe detector 200 (“off-axis”). When the subject's 206 eyes are open, thedifference between the images will highlight the pupils of the eyes.This is because the specular reflection from the retinas is detectedonly in the on-axis image. The diffuse reflections from other facial andenvironmental features are largely cancelled out, leaving the pupils asthe dominant feature in the differential image. This can be used toinfer the subject's 206 eyes are closed when the pupils are notdetectable in the differential image.

The amount of time the subject's 206 eyes are open or closed can bemonitored against a threshold in this embodiment in accordance with theinvention. Should the threshold not be satisfied (e.g. the percentage oftime the eyes are open fall below the threshold), an alarm or some otheraction can be taken to alert the subject 206. Other metrics, such as thefrequency or duration of blinking, may be used in other embodiments inaccordance with the invention.

Differential reflectivity off a retina of the subject 206 is dependentupon the angle 210 between light source 202 and the axis 208 of thedetector 200, and the angle 212 between the light source 204 and theaxis 208. In general, a smaller angle 210 will increase the retinalreturn. As used herein, “retinal return” refers to the intensity(brightness) that is reflected off the back of the subject's 206 eye anddetected at detector 200. “Retinal return” is also used to includereflection off other tissue at the back of the eye (other than or inaddition to the retina). Accordingly, angle 210 is selected such thatlight source 202 is on or close to axis 208. In this embodiment inaccordance with the invention, angle 210 is in the range ofapproximately zero to two degrees.

In general, the size of angle 212 is chosen so that only low retinalreturn from light source 204 will be detected at detector 200. The iris(surrounding the pupil) blocks this signal, and so pupil size underdifferent lighting conditions should be considered when selecting thesize of angle 212. In this embodiment in accordance with the invention,angle 210 is in the range of approximately three to fifteen degrees. Inother embodiments in accordance with the invention, the size of angles210, 212 may be different. For example, the characteristics of aparticular subject may determine the size of the angles 210, 212.

Light sources 202, 204 emit light that yields substantially equal imageintensity (brightness) in this embodiment in accordance with theinvention. Light sources 202, 204 emit light of different wavelengths inthis embodiment in accordance with the invention. Even though lightsources 202, 204 can be of any wavelength, the wavelengths are selectedin this embodiment so that the light will not distract the subject andthe iris of the eye will not contract in response to the light. Theselected wavelengths should be in a range that allows the detector 200to respond. In this embodiment in accordance with the invention, lightsources 202, 204 are implemented as light-emitting diodes (LEDs) ormulti-mode lasers having infrared or near-infrared wavelengths. Eachlight source 202, 204 may be implemented as one, or multiple, sources,where each such device is located at substantially the same angle fromthe axis 208.

FIG. 3 a illustrates an image generated with an on-axis light source inaccordance with the system shown in FIG. 2. The image shows an eye thatis open. The eye has a bright pupil due to a strong retinal returncreated by the on-axis light source 202. If the eye had been closed, ornearly closed, the bright pupil would not be detected and imaged.

FIG. 3 b depicts an image generated with an off-axis light source inaccordance with the system shown in FIG. 2. The image in FIG. 3 b may betaken at the same time as the image in FIG. 3 a, or it may be taken inan alternate frame (successively or non-successively) to the image ofFIG. 3 a. The image of FIG. 3 b illustrates a normal, dark pupil. If theeye had been closed or nearly closed, the normal pupil would not bedetected and imaged.

FIG. 3 c illustrates an image resulting from the difference between theFIG. 3 a image and the FIG. 3 b image. By taking the difference betweenthe images of FIGS. 3 a and 3 b, a relatively bright spot 300 remainsagainst the relatively dark background 302 when the eye is open. Theremay be vestiges of other features of the eye remaining in the background302. However, in general, the bright spot 300 will stand out incomparison to the background 3102. When the eye is closed or nearlyclosed, there will not be a bright spot 300 in the differential image.

FIGS. 3 a-3 c illustrate one eye of the subject 206. Those skilled inthe art will appreciate that both eyes may be monitored as well. It willalso be understood that a similar effect will be achieved if the imagesinclude other features of the subject 206 (e.g. other facial features),as well as features of the subject's 206 environment. These featureswill largely cancel out in a manner similar to that just described,leaving either a bright spot 300 when the eye is open (or two brightspots, one for each eye), or no spot(s) when the eye is closed or nearlyclosed.

Referring now to FIG. 4, there is shown a sensor in one embodiment inaccordance with the invention. In this embodiment, a sensor 400 isincorporated into detector 200 (FIG. 2), and is configured as acomplementary metal-oxide semiconductor (CMOS) imaging sensor. Sensor400, however, may be implemented with other types of imaging devices inother embodiments in accordance with the invention, such as, forexample, a charge-coupled device (CCD) imager.

A patterned filter layer is formed on sensor 400 using two differentfilters shaped into a checkerboard pattern. The two filters aredetermined by the wavelengths being used by light sources 202, 204. Forexample, in this embodiment in accordance with the invention, sensor 400includes regions (identified as 1) that include a filter material forselecting the wavelength used by light source 202, while other regions(identified as 2) include a filter material for selecting the wavelengthused by light source 204.

In the FIG. 4 embodiment, the patterned filter layer is deposited as aseparate layer of sensor 400, such as, for example, on top of anunderlying layer, using conventional deposition and photolithographyprocesses while still in wafer form. In another embodiment in accordancewith the invention, the patterned filter layer can be can be created asa separate element between sensor 400 and incident light. Additionally,the filter pattern can be configured in a pattern other than acheckerboard pattern. For example, the patterned filter layer can beformed into an interlaced striped or a non-symmetrical configuration(e.g. a 3-pixel by 2-pixel shape). The patterned filter layer may alsobe incorporated with other functions, such as color imagers.

Various types of filter materials can be used in the patterned filterlayer. In this embodiment in accordance with the invention, the filtermaterials include polymers doped with pigments or dyes. In otherembodiments in accordance with the invention, the filter materials mayinclude interference filters, reflective filters, and absorbing filtersmade of semiconductors, other inorganic materials, or organic materials.

FIG. 5 is a cross-sectional diagram of a detector in accordance with oneembodiment in accordance with the invention. Only a portion of thedetector is shown in this figure. Detector 200 includes a sensor 400comprised of pixels 500, 502, 504, 506, a patterned filter layer 508including two alternating filter regions 510, 512, a glass cover 514,and a dual-band narrowband filter 516. Sensor 400 is configured as aCMOS imager and the patterned filter layer 508 as two polymers 510, 512doped with pigments or dyes in this embodiment in accordance with theinvention. Each region in the patterned filter layer 508 (e.g. a squarein the checkerboard pattern) overlies a pixel in the CMOS imager.

When light strikes the upper surface of narrowband filter 516, the lightat wavelengths other than the wavelengths of light source 202 (λ₁) andlight source 204 (λ₂) are filtered out, or blocked, from passing throughthe narrowband filter 516. Thus, the light at visible wavelengthsλ_(VIS) and the light at wavelengths (λ_(n)) other than λ₁ and λ₂ arefiltered out in this embodiment, while the light at or near thewavelengths λ₁ and λ₂ transmit through the narrowband filter 516. Thus,only light at or near the wavelengths λ₁ and λ₂ pass through glass cover514. Thereafter, polymer 510 transmits the light at wavelength λ¹ whileblocking the light at wavelength λ². Consequently, pixels 500 and 504receive only the light at wavelength λ¹, thereby generating the imagetaken with the on-axis light source 202.

Polymer 512 transmits the light at wavelength λ₂ while blocking thelight at wavelength λ₁, so that pixels 502 and 506 receive only thelight at wavelength λ₂. In this manner, the image taken with theoff-axis light source 204 is generated. The shorter wavelength λ₁ isassociated with the on-axis light source 202, and the longer wavelengthλ₂ with the off-axis light source 204, in this embodiment in accordancewith the invention. The shorter wavelength λ₁, however, may beassociated with the off-axis light source 204 and the longer wavelengthλ₂ with the on-axis light source 202 in other embodiments in accordancewith the invention.

Narrowband filter 516 is a thin-film bulk dielectric stack filter inthis embodiment in accordance with the invention. Dielectric stackfilters are designed to have particular spectral properties. In thisembodiment in accordance with the invention, the dielectric stack filteris formed as a dual spike filter. The narrowband filter 516 (i.e.,dielectric stack filter) is designed to have one peak at λ₁ and anotherpeak at λ₂.

The narrowband filter 516 and the patterned filter layer 508 form ahybrid filter in this embodiment in accordance with the invention. FIG.6 depicts spectra for the polymer filters and the narrowband filtershown in FIG. 5. As shown in FIG. 6, the hybrid filter (combination ofthe polymer filters 510, 512 and the narrowband filter 516) effectivelyfilters out all light except for the light at or near the wavelengths ofthe light sources (λ₁ and λ₂). The narrowband filter 516 transmits anarrow amount of light at or near the wavelengths of interest, λ₁ andλ₂, while blocking the transmission of light at other wavelengths.Therefore, only the light at or near wavelengths λ₁ and λ₂ strikepolymer filters 510, 512 in the patterned filter layer 508. Thepatterned filter layer 508 is then used to discriminate between λ₁ andλ₂. Wavelength λ₁ is transmitted through filter 510 (and not throughfilter 512), while wavelength λ₂ is transmitted through filter 512 (andnot through filter 510).

Those skilled in the art will appreciate the patterned filter layer 508provides a mechanism for selecting channels at pixel spatial resolution.In this embodiment in accordance with the invention, channel one isassociated with the on-axis image and channel two with the off-axisimage. In other embodiments in accordance with the invention, channelone may be associated with the off-axis image and channel two with theon-axis image.

Sensor 400 sits in a carrier (not shown) in this embodiment inaccordance with the invention. The glass cover 514 typically protectsthe sensor 400 from damage and particle contamination (e.g. dust). Inanother embodiment in accordance with the invention, the hybrid filterincludes the patterned filter layer 508, the glass cover 514, and thenarrowband filter 516. The glass cover 514 in this embodiment is formedas a colored glass filter, and is included as the substrate of thedielectric stack filter (i.e., the narrowband filter 516). The coloredglass filter is designed to have certain spectral properties, and isdoped with pigments or dyes. Schott Optical Glass Inc., a companylocated in Mainz, Germany, is one company that manufactures coloredglass that can be used in colored glass filters.

Referring now to FIG. 7 a, there is shown a first method for fabricatinga dual spike filter in an embodiment in accordance with the invention.As discussed in conjunction with the FIG. 5 embodiment, the narrowbandfilter 516 is a dielectric stack filter that is formed as a dual spikefilter. Dielectric stack filters can include any combination of filtertypes. The desired spectral properties of the completed dielectric stackfilter determine which types of filters are included in the layers ofthe stack.

For example, a dual-spike filter can be fabricated by combining twofilters 700, 702. A band-blocking filter 700 filters out the light atwavelengths between the regions around wavelengths λ₁ and λ₂, while abandpass filter 702 transmits light near and between wavelengths λ₁ andλ₂. The combination of the two filters 700, 702 transmits light in thehatched areas, while blocking light at all other wavelengths. FIG. 7 bdepicts the spectrum for the dual spike filter shown in FIG. 7 a. As canbe seen, light transmits through the combined filters only at or nearthe wavelengths of interest, λ₁ (peak 704) and λ₂ (peak 706).

Referring now to FIG. 8 a, there is shown a second method forfabricating a dual spike filter in an embodiment in accordance with theinvention. A dual-spike filter can be fabricated in this embodiment bycombining a cut-off filter 800, a cut-on filter 802, and a band-blockingfilter 804. The combination of the three filters transmits light in thehatched areas, while blocking light at all other wavelengths. FIG. 8 bdepicts the spectrum for the dual spike filter shown in FIG. 8 a. As canbe seen, light transmits through the combined filters only at or nearthe wavelengths of interest, λ₁ (peak 806) and λ₂ (peak 808).

In some applications, such as in pupil detection, it is more desirableto use LEDs rather than lasers as light sources. LEDs are typicallysafer to use in eye detection compared with lasers. LEDs also have lowercoherence than lasers, which eliminates speckle. Additionally, more ofthe object is illuminated by LEDs because LEDs have broader divergence.And finally, LEDs are usually less expensive than lasers.

A narrowband light source can be created with an LED by placing anarrowband filter in front of, or on top of, the LED light source tonarrow the spectrum of the LED. FIG. 9 is a diagram of a light sourceand a narrowband filter in an embodiment in accordance with theinvention. Light source 900 is a light-emitting diode (LED) andnarrowband filter 902 is a single spike dielectric stack filter in theFIG. 9 embodiment. In other embodiments in accordance with theinvention, however, narrowband filter 902 may be configured as othertypes of filters. Furthermore, the dielectric stack filter may befabricated as, or including, a colored glass filter. And other lightsources, such as white light sources, may be used in other Embodimentsin accordance with the invention.

Referring now to FIG. 10, there is shown an illustration of the spectraof the light source and the combination of the light source and thenarrowband filter, as shown in FIG. 9. As can be seen, the spectrum(1000) of the light source 900 by itself is broader than the spectrum(1002) of the combination of the light source 900 and the narrowbandfilter 902. As discussed earlier, a narrowband light source can beconstructed with a broader spectrum light source 900 by forming orplacing a narrowband filter 902 on top of, or in front of, the broaderspectrum light source.

FIG. 11 is a diagram of a second system for pupil detection thatutilizes a narrowband filter with each light source and a hybrid filterwith a sensor in an embodiment in accordance with the invention. Similarreference numbers have been used for those elements that function asdescribed in conjunction with earlier figures. A detector 200 includes asensor 400, a patterned filter layer 508, a glass cover 514, and anarrowband filter 516. A lens 1100 captures the light reflected offsubject 206 and focuses it onto the top surface of the narrowband filter516 in detector 200.

The light source 202 includes a narrowband filter 902 a, while the lightsource 204 includes a narrowband filter 902 b. Narrowband filters 902 a,902 b have been fabricated to create filters having appropriate spectralproperties for the light sources 202, 204, respectively. As discussedearlier, the narrowband filters 902 a, 902 b allow the system to utilizelight sources that have broader spectra but may be safer and lessexpensive to use than narrower spectrum light sources. In otherembodiments in accordance with the invention, different types of filtersmay be used with light sources 202, 204. Examples of different filtertypes include, but are not limited to, cut-on filters and cut-offfilters.

The on-axis image is captured by detector 200 using light source 202,and the off-axis image is captured by detector 200 using light source204. The hybrid filter includes the patterned filter layer 508 and thenarrowband filter 516 in this embodiment in accordance with theinvention. The hybrid filter blocks out light at all wavelengths otherthan the wavelengths of the light sources 202, 204. Therefore, sensor400 detects only the light at the wavelengths of the light sources 202,204.

Referring now to FIG. 12, there is shown a diagram of a device that canbe used for pupil detection in one embodiment in accordance with theinvention. Device 1200 includes the detector 200, a number of on-axislight sources 202, and a number of off-axis light sources 204. Eachon-axis light source 202 is located at substantially the same angle fromthe axis of the detector 200 in this embodiment in accordance with theinvention. Similarly, each off-axis light source 204 is located atsubstantially the same angle from the axis of the detector 200.

The on-axis image is captured by detector 200 using light sources 202,and the off-axis image is captured by detector 200 using light sources204. The light sources 202, 204 are shown as being housed in the samedevice 1200 as the detector 200 in this embodiment in accordance withthe invention. In other embodiments in accordance with the invention,light sources 204 may be located in a housing separate from lightsources 202 and detector 200. Furthermore, light sources 202 may belocated in a housing separate from detector 200 by placing a beamsplitter between the detector and the object.

FIG. 13 is a diagram of a system for detecting transmission through anobject that utilizes a hybrid filter in another embodiment in accordancewith the invention. Similar reference numbers have been used for thoseelements that function as described in conjunction with earlier figures.A detector 200 includes a sensor 400, a patterned filter layer 508, aglass cover 514, and a narrowband filter 516.

A broadband light source 1300 transmits light toward a transparentobject 1302. The broadband light source 1300 emits light at multiplewavelengths, two of which will be detected by detector 200. In otherembodiments in accordance with the invention, the broadband light source1300 may be replaced by two light sources transmitting light atdifferent wavelengths.

A lens 1100 captures the light transmitted through the transparentobject 1300 and focuses it onto the top surface of the narrowband filter516. One image is captured by detector 200 using light that istransmitted at one wavelength, while a second image is captured bydetector 200 using light that is transmitted at the other wavelength.

The hybrid filter includes the patterned filter layer 508 and thenarrowband filter 516 in this embodiment in accordance with theinvention. The hybrid filter blocks out light at all wavelengths otherthan the two wavelengths of interest, allowing the sensor 400 to detectonly the light at the two wavelengths.

Although a hybrid filter has been described with reference to detectinglight at two wavelengths, λ₁ and λ₂, hybrid filters in other embodimentsin accordance with the invention may be used to detect more than twowavelengths of interest. FIG. 14 depicts spectra for polymer filters anda tri-band narrowband filter in an embodiment in accordance with theinvention. A hybrid filter in this embodiment detects light at threewavelengths of interest, λ₁, λ₂, and λ₃. The spectra 1400 and 1402 atwavelengths λ₁ and λ₃, respectively, represent two signals to beutilized by an imaging system. Light detected at wavelength λ₂ is usedto determine the amount of light received by the imaging system outsidethe two wavelengths of interest. The amount of light detected atwavelength λ₂ may be used as a reference amount of light detectable bythe imaging system.

A tri-band narrowband filter transmits light at or near the wavelengthsof interest (λ₁ λ₂, and λ₃) while blocking the transmission of light atall other wavelengths in this embodiment in accordance with theinvention. Polymer filters in a patterned filter layer then discriminatebetween the light received at wavelengths λ₁ λ₂, and λ₃. FIG. 15 depictsa sensor in accordance with the embodiment shown in FIG. 14. A patternedfilter layer is formed on sensor 1500 using three different filters. Forexample, in one embodiment in accordance with the invention, sensor 1500may include a red-green-blue color three-band filter pattern. Redcorresponds to regions 1, green to regions 2, and blue to regions 3 inthe figure. There are twice as many greens as other colors in thisembodiment because human perception of brightness depends most stronglyon the green range.

Referring now to FIG. 16, there is shown spectra for polymer filters anda quad-band narrowband filter in an embodiment in accordance with theinvention. A hybrid filter in this embodiment detects light at fourwavelengths of interest, λ₁, λ₂, λ₃, and λ₄. The spectra 1600, 1602 atwavelengths λ₁ and λ₃, respectively, represent two signals to beutilized by an imaging system. Light detected at wavelengths λ₂ and λ₄is used as a reference to determine the amount of light received by theimaging system outside the two wavelengths of interest.

A quad-band narrowband filter transmits light at or near the wavelengthsof interest (λ₁, λ₂, λ₃, and λ₄) while blocking the transmission oflight at all other wavelengths in this embodiment in accordance with theinvention. Polymer filters in a patterned filter layer then discriminatebetween the light received at wavelengths λ₁, λ₂, λ₃, and λ₄. FIG. 17depicts a sensor in accordance with the embodiment shown in FIG. 16. Apatterned filter layer is formed on sensor 1700 using four differentfilters. For example, in this embodiment in accordance with theinvention, sensor 1700 includes rows of filters 1 and 2 alternating withrows of filters 3 and 4. Filters 1 and 2 detect the light at wavelengthsλ₁ and λ₂, while filters 3 and 4 detect the light at wavelengths λ₃ andλ₄.

Hybrid filters in other embodiments in accordance with the invention maydetect any number of wavelengths of interest. For example, an n-bandnarrowband filter would transmit light at or near the wavelengths ofinterest. Regions within a patterned filter layer would alternatelyblock at least one (and up to (n−1)) of the wavelengths of interest. Forsystems that utilize a patterned filter layer having regions thattransmit two or more wavelengths of interest, mathematical computations,such as linear algebra, may be performed in order to determine theamount of light received at each wavelength of interest.

1. A detector for detecting light reflected off an eye of a subject, thedetector comprising: an unpatterned filter configured to pass thereflected light in at least two nonoverlapping wavelength bands and toblock the reflected light in wavelengths therebetween; a patternedfilter below the unpatterned filter having at least first and secondregions configured to discriminate between the light passed in the atleast two nonoverlapping wavelength bands by the unpatterned filter; anda light-detecting sensor below the patterned filter configured torespectively detect an amount of the light in the at least twononoverlapping wavelength bands discriminated by the patterned filter.2. The detector according to claim 1, wherein the light-detecting sensorcomprises a plurality of pixels and the at least first and secondregions of the patterned filter are arranged to correspond to therespective pixels.
 3. The detector according to claim 2, wherein a firstimage is generated from the pixels corresponding to the first regionsand a second image is generated from the pixels corresponding to thesecond regions.
 4. The detector according to claim 3, wherein the firstand second images are subtracted to form a differential image of the eyeand a pupil of the eye is detected from the differential image.